UC San Diego

Integral membrane transport proteins homologous to those found in the Transporter Classification Database (TCDB; www.tcdb.org) were identified and bioinformatically characterized by transporter class, family and substrate specificity in three ciliates, Paramecium tetraurelia (Para), Tetrahymena thermophila (Tetra), and Ichthyophthirius multifiliis (Ich). In these three organisms, 1326 out of 39,600 proteins (3.3%), 1017 out of 24,800 proteins (4.1%) and 504 out of 8,100 proteins (6.2%) integral membrane transport proteins were identified, respectively. Thus, an inverse relationship was observed between the % transporters identified and the number of proteins reported. Of these proteins in Para and Tetra, 41% were channels, 31% were secondary carriers, 26% were primary active transporters (mostly ATP-hydrolysis driven) and 2% were of unknown mechanism of action, but in Ich, the proportion of channels was reduced by 8%. Family representation was evaluated. With respect to substrates transported, Para has four times more inorganic anion transporters than Tetra, and Ich has non-selectively lost most of these. Tetra has more sugar and monocarboxylate transporters than Para, while Para seems to rely more on di and tricarboxylates. Ich may depend more on sugars than carboxylates. Para has about twice as many protein and lipid exporters than Tetra, and while Ich lost most of the former, it has retained most of the latter. These observations serve to characterize the transport proteins of these related ciliates, providing insight into their nutrition, metabolism and signaling mechanisms